Avaya BCM 2.0 IP Telephony Manual de usuario descargar pdf (Pagina 41)

Engineering guidelines 41

P0911590 Issue 02 Enterprise Edge 2.0 IP Telephony Configuration Guide

When a voice packet is delayed and does not arrive at the far-end in time to fit into

the voice stream going out of the far-end gateway, it is discarded, and the previous

packet is replayed. If the scenario occurs often, or twice in a row, the listener hears

reduced voice quality.

The jitter buffer hold time adds to the delay, so if the network has high jitter, the

effect is a long delay in the voice stream. For example, a network can have an

average delay of 50 ms and a variability of 5 ms. The network is said to have 5 ms

of jitter, a low figure. The jitter buffer hold time is only 5 ms, so the network total

delay is 55 ms.

If a network has a low average delay of 15 ms, but 10% of the time the delay goes

out to a 100 ms, while 90% of the time the delay is a brief 4 ms, the jitter buffer is

100 ms and the total network delay is 115 ms, a long delay. Network jitter can be

more important than average delay in many VoIP Gateway applications.

VoIP Gateway voice calls use an adaptive jitter buffer that changes the hold time

over the duration of the call. The installer or administrator configures the maximum

hold time.

VoIP Gateway fax calls use a fixed jitter buffer that does not change the hold time

over the duration of the call. Fax calls are more prone to packet loss. In conditions

of high jitter, increased delay (through the use of a deeper jitter buffer) is preferred.

To adapt, VoIP Gateway provides a separate jitter buffer setting for fax calls.

The voice jitter buffer parameters directly affect the end-to-end delay and audio

quality. IP telephony dynamically adjusts the size of the jitter buffer to adjust for

jitter in the network. The installer or administrator sets the starting point for the

jitter buffer.

Fax calls

The Enterprise Edge gateways support T.30 Group 3 fax calls. Fax calls

automatically use the G.711 codec and require the associated bandwidth.

As the gateway does not know in advance that a call carries a fax transmission, it

first establishes a voice channel. The voice channel can use G.729 or G.723.1 audio

compression. When detecting the answering fax machine’s CED tone, the

terminating gateway performs the following operations:

• Starts the procedure to revert the speech path to a G.711, 64 Kbit/s clear

channel.

• Disables the adaptive jitter buffer feature.

• Sets the hold time for the jitter buffer to the value indicated in the Local

Gateway settings to improve late IP packet tolerance.

The answering fax machine must produce its CED tone within 15 s of connection.

The terminating gateway turns off CED tone detection after 15 s to prevent false

tone detection during a voice call.

This method sets the following restrictions:

1 2 ... 36 37 38 39 40 41 42 43 44 45 46 ... 75 76

Sin comentarios

Avaya BCM 2.0 IP Telephony Manual de usuario Pagina 41